1. Field of Use
The present invention relates to a self-adjusting phase lock circuit.
2. Prior Art
In order to recognize digital information read out from a magnetic media, typically a magnetic disk, phase lock circuits are used. When a magnetic media is read, it produces, through suitable reading circuits, a sequence of electrical pulses having a variable modulated frequency relative to a nominal frequency.
The modulated frequency has two components, a logical or meaningful modulation component and a noise modulation component caused by changes of the magnetic media as to a nominal speed. In order to recognize the read out data, it is required to discriminate between the logical and noise components. This is achieved by a phase lock circuit which basically comprises a phase comparator and discriminator, an integrating network and voltage controlled oscillator. The phase comparator and discriminator receive as inputs, the electrical read out pulses, and a periodic signal having a frequency locked to the basic actual frequency of the read out pulses. The phase comparator and discriminator provide as outputs, a logic data signal and one or more phase error signals.
The integrating network receives as inputs, the phase error signals, and outputs a variable voltage signal related to the error between the average frequency of the read out pulses and the frequency of the periodic reference signal. The voltage controlled oscillator (VCO) provides as an output, a signal whose frequency varies as a function of the variable voltage received at its control input.
The phase comparator, phase discriminator and the integrating network form a feedback frequency control system for the voltage controlled oscillator in which the control voltage for the oscillator consists of the voltage output produced by the integrating network. The controlled variable is the variable frequency signal output generated by the oscillator which is applied as an input to the phase comparator and discriminator. Both phase comparators as well as voltage controlled oscillators are available on the market in the form of integrated circuits, such as the types of integrated circuits manufactured by Signetics Corporation designated by code SCB58459 and by Texas Instruments Inc. designated by code SN74LS629.
The voltage controlled oscillator is a critical element of this control system. Its input voltage/output frequency characteristics are largely variable from product unit to product unit, and are further affected by temperature as well as by a range voltage "VRANGE" applied as an adjustment input. The value of VRANGE defines the proportionality coefficient which relates the control voltage to the output frequency.
To provide correct and reliable operation of the control system, it is necessary to define a predetermined nominal working point for the oscillator. In other words, independently of the component used, a predetermined output frequency must correspond to a predetermined input control voltage, centered in the permissible control voltage range. Further, the set point must not drift with temperature changes.
This is accomplished by means of suitable adjusting circuits comprising trimming resistors and positive temperature coefficient thermistors (PCT) which provide a range voltage VRANGE to the adjusting input suitable for each particular unit which varies with the temperature so as to compensate for spread and drift in the operation of the used component. The adjusting operation inherent with the use of trimmers negatively affects the manufacturing costs of the phase lock circuits. Further, it compensates only in part for thermal drift.